Good you found a reasonable vision therapist.
Currently your son has -1.25D as the maximum recommended correction for distance. But if he got to this point without glasses that means that on a good day he still has the accommodation skills up to 1 diopter. So I’d be careful not to fix the current maximum correction in a pair of glasses and turn that into the minimum correction needed for the future.
I can understand if the vision therapist doesn’t recommend going completely glasses free, maybe there is a risk of one eye going dominant and one checking out of the vision game if there is a lot of squinting for distance. Squinting or focusing hard to figure out things is not the way of improving eyes.
What worked for others was to get a pair of glasses with lower correction than the max correction. (I don’t consider it undercorrection, since the measurement is typically done in a dark room, while your son most probably spends the majority of his time in better lights condition. The difference between dark room and good lights can easily be up to 0.5D in visual acuity)
Be without glasses whenever possible (especially for all close up activities) but if the kid asks for glasses for TV or for cinema or for watching a match live then allow wearing them. So no strain is needed but not wearing if not needed. Kids eyesight can restore pretty quickly and you may find that after some time there is less and less needs for the glasses.
Needless to say, parallel to that it would be important to reduced close up screen time (no tablet or phone at their short arms’ distance) and increase outdoor time - e.g. cycling or ball games. Anything where he has to change between focal distances, follow objects getting nearer or further away. That is the most natural gentle training for the eyes.